What could be causing the the battery light to stay on after replaceing the battery in a 2003 Ford Expedition?

The battery light comes on when the alternator is not charging. Changing the battery did nothing. Either put a gauge on the battery to see if it is charging, or while running, disconnet the positive cable from the battery. If the car dies that means the alternator is not charging. Most parts stores will check it for free. Oriely's , Autozone, etc.